In a large pot heat up the oil, add the bay leaf, onion and fry for a couple of minutes over a medium heat stirring occasionally.
Add the sauerkraut, stir and continue frying for another minute or so. Add 1/2 the amount of water, cover and bring to boil. Lower the heat and simmer for 20 minutes.
Add the rest of the ingredients (NOT the salt), including the remaining water, stir, cover, bring to boil again and continue cooking over a low heat for 50 minutes. DO NOT add salt until the soup is fully cooked as the sauerkraut is salty so taste the soup to decide how much salt it needs.
I recommend buying sauerkraut in a Polish delicatessen. Look for sauerkraut with no preservatives in it (other than salt), yellow rather than grey in colour.
Do not add any salt until the soup is ready and you can taste it to decide how much salt, if any, is needed.
To make meat kapusniak soup add the meat to the sauerkraut and cook for about 20 minutes, then continue cooking the soup according to the Instructions.
If you make this sauerkraut soup with meat add water rather than stock (both the meat as well as stock are salty). Adjust the amounts to suit your preference.
Spices: use cayenne or lots of black pepper for a bit of bite and smoked paprika (unless you are adding smoked meats to the soup). Some kapusniak recipes call for marjoram and caraway seeds. Use 1 teaspoon of each if you like.
Refrigerate for up to 3 days.
Freeze individual portions for up to 3 months.
To save time making this soup prepare the vegetables while the sauerkraut and onion are cooking.